Abstract

In order to reduce the maximum axial stress produced in the pipe string for mining manganese nodules from deep-sea floors by a pump-lift system, the string is assumed to be a two-stepped pipe string with larger diameter in the upper part. Moreover, a buffer and two pump-modules attached to the string are assumed to be equipped with a vibration absorber. Then, the longitudinal vibration and axial stress caused in the string by the heave-motion of the mining ship are analyzed theoretically for practical cases. The result shows that the maximum axial stress in the optimum stepped pipe-string with the buffer having the optimum vibration absorber is found to be smaller by about 42 percent than the one produced in the uniform pipe string with the same mass and no vibration absorbers.

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